1. Field of the Invention
The present invention relates to an active matrix display device and more particularly relates to an active matrix liquid crystal display device using digital gray scale. In addition, the invention relates to electronic equipment comprising such a display device.
2. Description of the Related Art
In recent years, as a flat panel display (FPD), an active matrix semiconductor display device leads the market. Above all, an active matrix liquid crystal display device in which liquid crystal is used for display medium (also known as electro-optic modulating layer) is widely used as a display device of electronic equipment such as a personal computer. In the active matrix liquid crystal display device, either analog gray scale in which the brightness of each pixel is continuously changed or digital gray scale in which the brightness of each pixel is discretely changed is used. Analog gray scale is realized, for example, by continuously changing a voltage applied to a liquid crystal cell allocated to each pixel and by continuously changing the light transmissivity of the liquid crystal cell. Area gray scale and time gray scale are included in digital gray scale. In area gray scale, a plurality of liquid crystal cells are allocated to each pixel and the brightness of each pixel is changed in accordance with a combination of liquid crystal cells which transmit light. Meanwhile, in time gray scale, a single liquid crystal cell is allocated to each pixel and the brightness of each pixel is changed by discretely changing light transmitting time of the liquid crystal cell in one frame. In addition, a color display is widely provided by using red (R), green (G) or blue (B) filter for each pixel.
FIG. 13 is a circuit diagram which shows a frame format of a conventional active matrix liquid crystal display device. As shown in FIG. 13, an active matrix liquid crystal display device 200 comprises a pixel matrix portion (also referred to as a liquid crystal display portion) 210, a signal line driver circuit 211, and a scan line driver circuit 212. In recent years, the pixel matrix portion 210, the signal line driver circuit 211, and the scan line driver circuit 212 of the active matrix liquid crystal display device 200 are formed on the same substrate by using low temperature poly-silicon thin film transistors (TFTs). Since such low temperature poly-silicon liquid crystal display device 200 can be easily reduced in size, it is particularly suitable for medium or small sized display panel of portable electronic equipment and the like. Furthermore, as the characteristics of low temperature poly-silicon TFTs are enhanced recently, circuits operated with a low voltage (for example 5V) in the liquid crystal display device 200, such as a CPU 213, a controller 214, a memory (not shown) can be made up of low temperature poly-silicon TFTs as well as the pixel matrix portion 210 and the driver circuits 211 and 212. When low temperature poly-silicon TFTs are used for these low-voltage circuits, it is desirable to shorten the gate length in order to improve frequency characteristics and increase element density. However, in the case of shortening the gate length, short channel effect easily occurs, and the characteristics of TFTs vary easily by the drain voltage. Therefore, it is necessary for instance to make a gate insulating layer as thin as possible in order to suppress the short channel effect. For example, it is preferable that a TFT of 5 V has a gate of 2 μm or less in length and a gate insulating layer of 50 nm or less in thickness.
In the pixel matrix portion 210, a signal line 230 and a scan line 231 are arranged in matrix, and a pixel TFT 242 is disposed at an intersection of the signal line 230 and the scan line 231. For the pixel TFT 242, a field effect transistor (FET) is used in general. The gate, source and drain of each TFT 242 are connected to the corresponding scan line 231, the signal line 230 and a pixel electrode 222, respectively. It is to be noted that the signal line 230 and the scan line 231 are respectively connected to the source and gate of the corresponding TFT 242, thus they may be referred to as a source signal line and a gate signal line, respectively.
A counter electrode 223 is arranged so as to face a plurality of pixel electrodes 222, and a liquid crystal 224 is arranged between the pixel electrodes 222 and the counter electrode 223. |In other words, a liquid crystal cell 221 is composed of the pixel electrode 222, the counter electrode 223 and the liquid crystal 224. It is to be noted that although separate liquid crystals 224 seem to be provided in each pixel electrode 222 in FIG. 13, the liquid crystal 224 is ordinarily used as a single member which extends across a plurality of pixel electrodes 222, as well known to those skilled in the art. The same is equally true for the counter electrode 223.
In general, the liquid crystal cell 221 which is composed of the pixel electrode 222, the counter electrode 223, and the liquid crystal 224 interposed therebetween cannot have large electrostatic capacity. Therefore, a storage capacitor 225 is provided in the vicinity of the pixel electrode 222 in order to store electric charge. Although not shown, the TFT 242 and the pixel electrode 222 in the pixel matrix portion 210, and the driver circuits 211 and 212 are ordinarily provided on the same substrate (also referred to as an active matrix substrate or an element substrate). On the other hand, the counter electrode 223 is provided on another substrate (also referred to as a counter substrate). The liquid crystal 224 is interposed between the two substrates.
When a potential (a selective signal) is applied to the scan line 231 so that a voltage between the gate and source of the TFT 242 exceeds the threshold voltage, the TFT 242 is turned on. Then, the drain and source of the TFT 242 are short circuited. The potential applied to the signal line 230 is transmitted to the pixel electrode 222, and the liquid crystal cell 221 and the storage capacitor 225 are charged in accordance with that potential. When the TFT 242 is turned off, there is no conductivity between the drain and source of the TFT 242. The electric charge stored in the liquid crystal cell 221 and the storage capacitor 225 is held until the TFT 242 is turned on. Light transmissivity of the liquid crystal 224 varies depending on whether a voltage is applied or not. Therefore, the brightness of each liquid crystal cell 221 can vary by controlling a potential Vpix of the pixel electrode 222 and a potential Vcom of the counter electrode 223.
When area gray scale is used in the liquid crystal display device 200, for example two adjacent liquid crystal cells 221 are allocated to one pixel. In such a case, the brightness of the pixel can vary with four levels in accordance with a combination of on/off of the two liquid crystal cells 221 (4-level gray scale). When the number of liquid crystal cells 221 to be allocated to each pixel is increased, the brightness of each pixel can vary with multi-level gray scale. The liquid crystal cells 221 having different areas may be allocated to each pixel. Generally and preferably, when k liquid crystal cells E1, E2, . . . , Ek are allocated to one pixel (that is, the number of indicator bits is k), the areas of each liquid crystal cell E1, E2, . . . , Ek are designed so as to be E1=1×E0, E2=2×E0, . . . , Ek=2(k-1)×E0, when the smallest area of the liquid crystal cell is set as E0. By changing the combination of these areas, the brightness of the pixel can vary with 2k-level gray scale as the brightness corresponding to E0 is the smallest unit. In addition, when one liquid crystal cell 221 is allocated to each pixel, digital gray scale can also be used by discretely changing light transmitting time of the liquid crystal cell 221 in one frame of video signal (time gray scale). In this case, k light transmitting time lengths T1, T2, . . . , Tk (the total of T1 to Tk is less than one frame period) are designed so as to be T1=1×T0, T2=2×T0, . . . , Tk=2(k-1)×T0, when the shortest transmitting time length is set as T0. By changing the combination of these lengths, the brightness of the pixel can vary with 2k-level gray scale as the brightness corresponding to T0 is the smallest unit. It is to be noted that in the case of using time gray scale, one frame period is divided into a plurality of subframe periods (pairs of scan period and fly-back period) in order to scan for selecting light transmitting state or non-light transmitting state of the liquid crystal cell in each light emitting time.
In general, the liquid crystal 224 has hysteresis with respect to an applied voltage. Therefore, when a direct current voltage is applied to the liquid crystal 224 for a long period, deterioration such as image persistence is caused. To prevent such image persistence, an electric field in a reverse direction is applied to the liquid crystal 224 at every predetermined period so that the average of voltages applied to the liquid crystal 224 is zero. This driving method is called the inversion drive. In order to perform the inversion drive, as shown in FIG. 14, the potential Vcom of the counter electrode 223 is kept stable, and the polarity of the potential Vpix applied to the pixel electrode 222 (that is, signal line potential) is reversed at every predetermined period (per frame period, for example) based on the potential Vcom of the counter electrode 223. For instance, when the potential Vcom of the counter electrode 223 is 8 V and the potential Vpix of the pixel electrode 222 oscillates between 3 and 13 V, a voltage applied to the liquid crystal 224 is switched between +5 and −5 V. It is to be noted that such inversion drive can be applied to other display medium having hysteresis with respect to an applied voltage as well as the liquid crystal.
In such a driving method, however, amplitude range of a signal line potential is twice as large as a voltage (absolute value) applied to the liquid crystal 224. Therefore, it is required to increase withstand voltage of the signal line driver circuit 211. Further, the gate potential of each TFT 242 varies depending on the source potential. Accordingly, as amplitude range of the signal line potential applied to the source is increased, amplitude range of the gate potential is also increased (for example, from 0 to 16 V). It is thus necessary to increase withstand voltage of the scan line driver circuit 212 to which the gate is connected. For instance, TFTs used for these driver circuits 211 and 212 have preferably a gate of 5 μm or more in length and a gate insulating layer of 100 nm or more in thickness. Moreover, an LDD structure or a gate overlap LDD structure (GOLD structure) is required, hence the manufacturing cost is increased.
As described above, low-voltage TFTs used for the CPU 213 and the controller 214 have desirably a gate of 2 μm or less in length and a gate insulating layer of 50 nm or less in thickness. However, when using the driving method shown in FIG. 14, such TFTs cannot be used for the driver circuits 211 and 212. Accordingly, it is necessary to fabricate two types of TFTs: high-voltage TFTs used for the driver circuits 211 and 212, and low-voltage TFTs used for the CPU 213 and the controller 214. Different processes are required for fabricating these TFTs, thus manufacturing processes and costs are increased.
Another driving method is described with reference to FIG. 15. The potential Vcom of the counter electrode 223 is switched between a high level common potential VcomH and a low level common potential VcomL per frame period, for example. Then, the signal line potential Vpix applied to the pixel electrode 222 varies depending on the potential Vcom of the counter electrode 223 (called AC drive). By using this driving method, amplitude range of the potential Vpix of the pixel electrode 222 (signal line potential) can be reduced by half (that is, the same as a voltage applied to the liquid crystal 224) as compared with using the inversion drive shown in FIG. 13. Hence, withstand voltage of the scan line driver circuit 212 can be reduced as well as that of the signal line driver circuit 211. Accordingly, withstand voltage of TFTs used for these driver circuits 211 and 212 can also be reduced, which results in a reduction in the manufacturing cost. In such AC drive, distortion of the image caused by switching the potential Vcom of the counter electrode 223 is necessarily reduced as much as possible. In view of the foregoing, it is suggested that the potential Vcom of the counter electrode 223 is switched and scanned (a potential of the pixel electrode 221 is set for all the pixels) during a period in which a light source such as a back light is turned off (Patent Document 1). This driving method allows to reduce withstand voltage of the driver circuits 211 and 212, but has problems as described below.
For example, in the liquid crystal display device 200, the liquid crystal 224 is switched from a transmissive state to a non-transmissive state when a voltage of 5V is applied. The potential Vcom of the counter electrode 223 and the potential Vpix of the signal line 230 are alternately operated with a voltage of 0 and 5 V (that is, VcomL=0 V and VcomH=5 V in FIG. 15). In such a case, when the potential Vcom of the counter electrode is 0 V in a frame, a voltage of 5 V has to be applied to the liquid crystal 224 in order to obtain a black display in one of the liquid crystal cells 221. Accordingly, the potential Vpix of the corresponding signal line (the potential of the pixel electrode 222) has to be at 5 V. As a result, a voltage of 5 V is charged across the corresponding storage capacitor 225. The potential Vcom of the counter electrode 223 is switched to 5 V in the next frame. However, when data of the liquid crystal cell 221 (voltage across the storage capacitor 225) has not been rewritten yet, electric charge stored in the storage capacitor 225 (or voltage across the storage capacitor 225) is stored. Therefore, the voltage across the storage capacitor 225 is added to the potential Vcom of the counter electrode 223, then the potential Vpix of the pixel electrode 222 is raised to 10 V. Accordingly, the pixel electrode 222 and elements connected thereto (including the pixel TFT 242) require a withstand voltage of 10 V or more, and the manufacturing cost is thus increased.
Further, since the light source is turned off during scanning and is turned on after scanning, emitting time of the light source is made shorter especially when the number of pixels is increased and it takes much time to scan. Thus, it is difficult to obtain a display with enough brightness.
It is suggested that instead of the storage capacitor, a memory circuit is provided between each pixel TFT and the corresponding pixel electrode, and either a high level power supply potential or a low level power supply potential is directly supplied to the pixel electrode in accordance with data stored in the memory circuit (Patent Document 2).
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-287708
[Patent Document 2]
Japanese Patent Application Laid-Open No. H07-199157